Progressively scanned television receivers and monitors have been proposed wherein the horizontal scan rate is multiplied, e.g., doubled, and for each line of incoming video a number, e.g., two, of lines (being either replicated or interpolated from incoming video lines) are displayed thereby providing a displayed image having reduced visibility of line structure.
In a typical progressively scanned receiver, wherein the added lines are replicas of the incoming video lines, each line of video is stored in one of two memories. As a first of the memories is being written with the incoming video signal at a standard line rate, the second of the memories is read two times at twice the standard line rate thereby providing two lines of "speed-up" (time-compressed) video within one standard line interval. The second memory output is applied to a display having a doubled horizontal sweep rate synchronized with read-out of the memory thereby doubling the number of displayed lines of video signal. An example of such a progressively scanned receiver is described in U.S. Pat. No. 4,415,931 entitled TELEVISION DISPLAY WITH DOUBLED HORIZONTAL LINES which issued Nov. 15, 1983 to R. A. Dischert.
It has been recognized that a desirable reduction of certain artifacts (e.g., inter-line flicker, line break-up with motion, etc.) may be obtained in a progressively scanned receiver by interpolating the added lines of the video signal from the original signal. This may be done either before or after "speed-up" (i.e., time compressing) of the video signal in the memory. An example of a progressively scanned display system in which the additional scan lines are obtained by interpolation from the original scan lines prior to time compression or video "speed-up" is described by K. H. Powers in U.S. Pat. No. 4,400,719 entitled TELEVISION DISPLAY SYSTEM WITH REDUCED LINE SCAN ARTIFACTS which issued Aug. 23, 1983. An alternative of providing interpolation subsequent to speed-up of the video signal is described by Yasushi Fujimura et al. in UK Patent Application No. 2,111,343A published June 29, 1983.
It has been further recognized that additional reduction of visible artifacts may be obtained in a progressively scanned receiver by using combination of line stores and field or frame stores to provide video time compression. An arrangement in which the field rate is doubled and the number of lines per field is also doubled is described by Lord et al. in U.S. Pat. No. 4,322,750 entitled TELEVISION DISPLAY SYSTEM which issued Mar. 30, 1982.
In such "line" or "field" or "frame" types of progressive scan systems the horizontal sweep rate is increased by the same "speed-up" factor (e.g., 2:1 or 4:1) as the video signal. For example, if the line rate is doubled or quadrupled in the video processor for display, then the horizontal sweep rate must also be doubled or quadrupled. This undesirably reduces the horizontal retrace time. The effects of the reduction in retrace time are numerous. In a double scanning system, for example, the deflection supply voltage for a given yoke winding must also be doubled to produce the required change in yoke current. The peak retrace voltage will also be doubled as all deflection voltage waveforms are increased by a given factor in amplitude and compressed in time by the same factor. Thus, the turn-off power losses in the horizontal output transistor are increased by a factor of eight. The reduced turn-off time and doubled voltage cause the rate of voltage rise (dv/dt) at turn-off to be four times greater. The energy lost per turn-off event is thus four times normal and the rate of turn-off events is twice normal resulting in an eight fold increase in the turn-off power loss. If one were to attempt to solve this problem by reducing the yoke impedance by a factor of four, the yoke voltages would be normal but the currents would be doubled. Power losses are unchanged by such impedance scaling.